408 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
organization of the chromosomes with respect to the chromomeres 
which they contain, this interchange of material must involve exactly 
equivalent portions of the two chromosomes. The chromosomes of 
the reduction division shown at 
i F may not, therefore, be identi- 
cal with the four originally 
present in A, but may represent 
various combinations of portions 
of both members of a particular 
pair of chromosomes. The re- 
U sults of such interchange between 
Fic. 73.—Diagram of chromatin inter- members of homologous pairs of 
change between homologous members of 2 chromosomes is shown in Fig. 73. 
pair of chromosomes. (From Babcock and At the left is shown a pair of 
Clausen, after Miiller.) : : 
chromosomes, one in outline, the 
other in full black. In the middle the steps in chromatin interchange 
are diagrammed and finally at the right this interchange results in 
a pair of chromosomes each of which is made up of parts of both 
members of the original pair of chromosomes. Various combinations 
may result depending on the points at which interchange takes place, 
but in every case the exchange involves corresponding portions of 
the two chromosomes. 
Independent distribution of chromosomes.—In Fig. 74 are illus- 
trated diagrammatically the chromosomes of Drosophila, with particu- 
lar reference to their size and form relations and to their character- 
istic pairing in the cell. One member of each of these pairs of chro- 
mosomes was contributed by the female parent and one member by 
the male parent. In the reduction divisions these chromosomes are 
separated so that each germ cell contains one member of each pair of 
chromosomes. The simplest condition which could obtain is that of 
independent distribution in each pair of chromosomes such that the 
particular member of one pair which went to a given pole of the reduc- 
tion spindle would have no influence on the distribution of the mem- 
bers of any other pair. Such independent distribution of chromo- 
somes appears to be actually the type followed in reduction. As a 
consequence the germ cells contain various combinations of chromo- 
somes with respect to their original parental derivation. In Fig. 74 
the types of combinations of maternal and paternal chromosomes and 
their mode of derivation in Drosophila are shown diagrammatically. 
Two germ cells, one from the female with the chromosomes in outline, 
